As a longarm quilter, I sometimes shudder when a pieced back comes to my studio with the top. The best back of course is a fat back (extra wide fabric) so there is no seam at all. But the selection in the fabric stores are limited and sometimes you just can’t find what you like, so you need to piece the back with normal 45” fabric. No problem, except when there is!
The next best solution is to have as few seams as possible and they all run the same direction, preferably with the grain. But if that’s not possible, and you need to cut on the cross grain, you need to be sure to cut at a 90 degree angle to the grain, so it will stay as straight as possible. Sometimes you need to piece some smaller pieces with the seams running perpendicular to the longer seams. That’s OK as long as you are careful with the cutting.
If you cut a bit off the grain, you are now into the bias and we all know that the bias stretches. When the back is loaded onto the longarm frame, as long as all the fabric grains are straight, they will stretch taut on the frame and stay flat. But if you are on the bias at all, the back will sag at that seam as the machine moves over the quilt sandwich and then tucks are formed which often is not felt or seen until the quilting is finished. Then a lot of ripping, steaming, pinning, and re-quilting ensues to take out the tucks to get it to lay flat. That costs a lot of time and creates a lot of aggravation for the quilter.
Another issue with pieced backs are they often are brought in not being square. One side of the seam is longer than the other. Sometimes the two pieces were not the same to begin with, but often they are because a long piece is cut in half and then the two halves are sewn together on the perpendicular sides to the cut. But if you just hold them together and sew a long seam, the feed dogs of your machine will feed the bottom faster than the top and you end up with the bottom edges not together. At this point it could be trimmed, but one side is stretched more than the other and the seam does not lay perfectly flat. So what I suggest is that you pin that long side at least a foot apart to keep both the top and bottom edges even. I also recommend a half inch seam and to press it open. This then allows the back to be very flat when quilted and sometimes not even noticeable when finished.
Why do we want the backs to be square when they’re just going to be trimmed afterward? It’s because they need to be square to be mounted on the frame. I actually fold in half to find the center on opposites sides and then baste long zippers on to attach the back to the frame. When the back isn’t square, then I have more work to do, to get it square. If it’s significant, it’s an added charge to the total cost of getting it quilted.
I hope this blog has added awareness to how to prepare a back for longarm quilting. I know that all longarmers out there appreciate and nice square back!